Tuberculin Skin Test

Test Overview

A tuberculin skin test (also called a Mantoux tuberculin test) is done to see if
you have ever been exposed to
tuberculosis (TB). The test is done by putting a small
amount of TB protein (antigens) under the top layer of skin
on your inner forearm. If you have ever been exposed to the TB bacteria (Mycobacterium tuberculosis), your skin will react to the
antigens by developing a firm red bump at the site within 2 days.

The TB antigens used in a tuberculin skin test are called purified
protein derivative (PPD). A measured amount of PPD in a shot is put under the
top layer of skin on your forearm. This is a good test for finding a TB
infection. It is often used when symptoms, screening, or testing, such as a
chest
X-ray, show that a person may have TB.

A
tuberculin skin test cannot tell how long you have been infected with TB. It
also cannot tell if the infection is
latent (inactive) or is active and can be passed to
others.

Why It Is Done

A tuberculin skin test is done to find
people who have tuberculosis (TB), including:

People who have been in close contact with
someone known to have TB.

Health care workers who are likely to be
exposed to TB.

People with TB symptoms, such as an ongoing cough,
night sweats, and unexplained weight loss.

How To Prepare

Have symptoms of tuberculosis (TB), such as an
ongoing cough, night sweats, or unexplained weight loss.

Have had
a severe reaction to a tuberculin skin test in the past.

Have had
TB in the past.

Have risk factors for TB, which are things that increase your risk. Risk factors for TB include:

Contact with a person who has
TB.

A job as a health care worker that may cause you to be exposed
to people with TB.

Having lived or traveled in a country where TB
is common.

Have been given a TB vaccination. The vaccine
contains a bacteria called BCG (bacille Calmette-Guérin) that is
closely related to the bacteria that cause TB.

Have been treated
with medicines, such as
corticosteroids, that can affect your
immune system.

Are infected with HIV.

Have a skin rash that may make it
hard to read the skin test.

Talk to your doctor about any concerns you have regarding
the need for the test, its risks, how it will be done, or what the results will
mean. To help you understand the importance of this test, fill out the
medical test information form(What is a PDF document?).

How It Is Done

For a tuberculin skin test, you sit
down and turn the inner side of your forearm up. The skin where the test is
done is cleaned and allowed to dry. A small shot of the TB antigen (purified
protein derivative, or PPD) is put under the top layer of skin. The fluid makes
a little bump (wheal) under the skin. A circle may be drawn around the test
area with a pen.

Do not cover the site with a bandage. You must see
your doctor 2 to 3 days after the test to have the skin test checked.

How It Feels

You may feel a quick sting or pinch from
the needle.

Risks

There is a very slight risk of having a severe
reaction to the tuberculin skin test, especially if you have had tuberculosis
(TB). An
allergic reaction can cause a lot of swelling and pain
at the site. A sore may be present.

You cannot get a TB infection
from the tuberculin skin test, because no live bacteria are used for the
test.

A tuberculin skin test is safe for pregnant or breast-feeding women.

After the test

Some redness at the skin test site
is expected. The site may itch, but it is important that you do not scratch it,
since this may cause redness or swelling that could make it hard to read the
skin test. If itching is a problem, put a cold washcloth on the site and then
dry it.

A strong positive reaction may cause mild pain. Talk to
your doctor if you have:

Results

A tuberculin skin test is done to see if
you have ever had
tuberculosis (TB) (infection with Mycobacterium tuberculosis).

Redness alone at the skin test
site usually means you have not been infected with TB bacteria. A firm red bump
may mean you have been infected with TB bacteria at some time. The size of the
firm bump (not the red area) is measured 2 to 3 days after the test to
determine the result. Your doctor will consider your chance of having TB when
looking at the skin test site.

Results of the test depend on your risk for TB. If you are in a high-risk group, a smaller bump is considered a sign of infection. People at low risk for having TB need to have a larger bump to be diagnosed with a TB infection.

Three levels of risk have been defined:

High-risk group includes
people who have
HIV, those who have had
close recent contact with a person who has an active TB infection, and those
who have symptoms or a chest
X-ray that shows TB. Other people at high risk for
tuberculosis include those who take medicines that contain
corticosteroids for a long time or people taking biologics (used to treat
rheumatoid arthritis and
Crohn's disease).

Moderate-risk group includes people who have recently moved
from or traveled in a country with a high rate of TB; those who use illegal
drugs by injection (intravenous drug users); people who
live in nursing homes; workers in hospitals, nursing homes, schools, and
prisons; children younger than 4 years old; children (ages 4 to 18) who are
exposed to high-risk adults; and homeless people. Others at moderate risk for
having tuberculosis include people who are 10% or more below their ideal body
weight and people who have
kidney failure,
diabetes,
leukemia, cancer, or those who have had part of their
stomach removed (gastrectomy).

Low-risk group includes people who do not have any possible exposure to TB listed
in the other risk groups.

A positive reaction usually remains visible for about 1
week.

Mantoux tuberculin skin test

Normal (negative
results):

No firm bump forms at the test
site, or a bump forms that is smaller than
5 mm (0.2 in.).

Abnormal (positive results):

A firm bump that is
5 mm (0.2 in.) in size suggests a
TB infection in people who are in a high-risk group.

A firm bump that is
10 mm (0.4 in.) in size suggests a
TB infection in people who are in a moderate-risk group.

A firm bump that is
15 mm (0.6 in.) in size suggests a
TB infection in people who are in a low-risk group.

A positive tuberculin skin test does not mean you have a
contagious (active) infection. The test cannot tell if the infection is active
or inactive (latent TB). It also cannot tell the difference between a TB
infection and a TB vaccination (BCG vaccination). More tests—such as a
chest X-ray, a sputum
culture, or both—are usually done to see if you have
an active TB infection.

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

A BCG (bacille Calmette-Guérin) vaccination. If
you have had a BCG vaccination, you may have a positive PPD skin
test even though you don't have TB.

Taking medicines that suppress the
immune system, such as
corticosteroids.

Conditions that weaken the immune system, such as
HIV infection or cancer. The result also may be affected if a person is
severely malnourished.

Some vaccinations for infections, such as
measles,
mumps,
rubella,
polio, or
chickenpox, given within 6 weeks before the tuberculin
test. A recent infection with one of these viruses can also interfere with test
results for a short period of time. The skin test also may be positive if the
person has an infection caused by a mycobacterium other than the one that
causes TB.

A very recent TB infection. It takes 2 to 10 weeks for
the immune system to react to TB bacteria.

Age younger than 3
months old. A baby's immune system is not fully developed at this
age.

A "booster effect." This tends to occur in people who get
regular TB skin tests, such as health care workers. The booster effect is a
weak or no reaction to one TB skin test followed by a strong reaction from a
second test. But the strong reaction to the second test does not mean that the
person has just become infected with TB. Instead, the reaction may be because
of:

A TB infection that occurred long ago. Over time, a
person's immune system tends to stop reacting strongly to the TB bacteria. So
the first TB skin test stimulates (boosts) the immune system, which then reacts
strongly to the second skin test.

The BCG vaccine, which contains
bacteria similar to the bacteria that cause TB. A person's immune system does
not react as strongly to the vaccine bacteria, and this tends to diminish over
time. So the person's immune system is boosted by the first skin test and
reacts strongly to the second test.

Infection with bacteria similar
to Mycobacterium tuberculosis. This may also cause a
first TB skin test to boost the immune system, allowing it to react strongly to
a second test, even though a new TB infection has not occurred.

What To Think About

The results of a tuberculin skin test alone
cannot confirm an active TB infection. Other tests, such as a chest X-ray, sputum cytology, and
sputum culture, may be done to confirm an active TB
infection when a skin test is positive. A person who has a positive skin test
or chest X-ray, but no TB symptoms, is usually thought to have a TB infection
that cannot be passed to others (latent TB).

Among hospital workers or
others who have periodic skin tests, a second test done within a few weeks of a
negative test may be positive, even though the person was not infected between
the two tests. These results (called the booster effect) may indicate a TB infection that occurred a long time ago or a
previous BCG vaccination.

About 5% to 10% of people (1 to 2 people out of 20) who
have inactive (latent) TB will develop active TB at some time in their lives.1 The chance of developing
active TB is higher in children, older adults, and people with an
impaired immune system.

No more tests are
needed for a person with a negative tuberculin skin test who has no symptoms of
active infection and no history of being exposed to TB.

Some people
do not react to a tuberculin skin test even if they have tuberculosis.
Conditions such as active TB, cancer, or
acquired immunodeficiency syndrome (AIDS) do not always
respond normally to the TB antigens. In these cases, other skin tests may be
done. If there is a skin reaction, then the tuberculin skin test is probably
correct. But if there is no reaction, the person's immune system is likely to
be too weak to respond normally to the tuberculin skin test.

Rapid
blood tests to diagnose TB have been approved by the U.S. Food and Drug
Administration (FDA). These tests may be used instead of a tuberculin skin
test. A rapid blood test may be able to tell if a person reacted to a skin test
because of an active TB infection or a previous BCG vaccination. Rapid blood tests are also called interferon-gamma release assays (IGRAs).

Rapid sputum tests that can detect TB bacteria in sputum have been approved by the FDA. These tests, called nucleic acid amplification tests (NAAs) can provide results within 24 hours. But they are done only when a person is strongly suspected of having TB.

References

Citations

Other Works Consulted

American Thoracic Society, Centers for Disease Control
and Prevention, Infectious Diseases Society of America (2003). Treatment of
tuberculosis. American Journal of Respiratory and Critical Care Medicine, 167(4): 603–662.

Centers for Disease Control and Prevention (2005).
Guidelines for using the QuantiFERON®-TB test for
diagnosing latent Mycobacterium tuberculosis infection.
MMWR, 54(RR-15): 49–55.

Division of Tuberculosis Elimination, National Center
for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention (2009). Questions and Answers About TB. Available online:
http://www.cdc.gov/tb/publications/faqs/default.htm.

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